Angela Merkel's efforts to entice Germany's Social Democrats into government have hit a roadblock in the form of the opposition's top female figure.

Hannelore Kraft, the premier of Germany's biggest state with 18 million people, is often talked about as the potential Left-wing successor to Germany's first female leader.

As a rising star in the SDP, Ms Kraft will this week play an influential role at a gathering of 200 party officials as it considers Mrs Merkel's coalition offer.

Speaking in her hometown of Düsseldorf, Ms Kraft declared political parties should not be up for sale to the "majority buyer." Any coalition deal should come at the price of changing the terms under which Mrs Merkel operates her government, she said, adding democracy needs strong oppositions, not just stable government.

"Although the SPD is ready to hold talks with the [CDU] Chancellor Angela Merkel but these could go in either direction," said Ms Kraft. "We are not going to wave flags for a grand coalition."

Concerns exists throughout the SPD over the damage that acting as Mrs Merkel's junior partner would inflict on its electoral appeal.

Party leaders in Berlin have called on the Chancellor, who fell just five seats short of a majority, to name her terms for talks. But provincial officials, who exercise real power in Germany's federal politics, have lined up alongside Ms Kraft to take much more hostile positions.

Mrs Merkel and Sigmar Gabriel, the SPD chairman, have suspended contacts until a summit in its headquarters on Friday, the outcome of which is expected to authorise negotiations but set out conditions that look impossible for the Chancellor to meet.

The stakes will be higher for Ms Kraft than almost anyone else in attendance. She led the SPD to a comeback victory in North Rhine-Westphalia in 2010 after it lost massive support to the hard-Left parties as a result of the last grand coalition in 2005. Since then she has actively promoted herself as the front-running chancellor candidate in the 2017 election.

"It's alright for the people who will have ministerial Mercedes turning up outside their flats in a few weeks but there are plenty like Hannelore Kraft who would say four years of us carrying the can for Merkel-ish politics will push us below 20 per cent," said Dennis McShane, the former Labour MP who knows Ms Kraft. "She has seen the damage a grand coalition can do and has to be very critical and hostile now in case the worst happens."

A graduate of London's Kings College and mother-of-one, Ms Kraft can already claim to be Germany's best-known female leader after Mrs Merkel.

Despite leading a minority government, she has implemented a series of budget cuts that have marked her out as a reformist.